A color filter used in a conventional liquid crystal display or the like is manufactured by methods such as a pigment dispersion method, a dyeing method, an electrodeposition method, a printing method, and an ink-jetting method.
Recently, the pigment dispersion method is mainly used. However, this method comprises steps applying a photopolymerizable colored composition to a substrate, exposing the substrate to radiation energy through a pattern-mask, dissolving the non-required portion, and washing, which are repeated for each red, green, and blue pixels, thereby to form a color filter. Thus, the pigment dispersion method has a disadvantage of high production cost. On the other hand, the ink-jet printing method, which can form three pixels in one step, has received much attention because of its simplicity and low cost due to high efficiency of coloring materials.
For example, although methods of forming a color filter by the ink-jet printing method are disclosed in Japanese Unexamined Patent Application Publication Nos. S59-75205 and S61-245106, the color filters obtained by these methods are inferior in heat resistance and solvent resistance because the coloring material consists of a dye. On the other hand, various methods using a resin and a pigment have been proposed as the method of producing a colored layer having excellent heat resistance and solvent resistance.
For example, Japanese Unexamined Patent Application Publication No. H5-224007 discloses a color filter using ink-jet printing ink comprising a melamine resin and a colorant, and Japanese Unexamined Patent Application Publication No. H8-171010 discloses a color filter using thermosetting or photocurable ink-jet printing ink containing an acrylamide polymer.
In addition, Japanese Unexamined Patent Application Publication No. H10-17813 discloses a color filter using ink-jet ink consisting mainly of a melamine resin, a polycarboxylic acid derivative, and an amine stabilizer, and Japanese Unexamined Patent Application Publication No. H7-188596 discloses ink for thermosetting ink-jet recording using a thermosetting resin and a specific amine as a dispersant.
In general, as a curable resin, a composition consisting of an acryl resin containing a carboxylic group or epoxy group, a multifunctional monomer having an ethylenically unsaturated group, which functions as a crosslinking agent, and a thermal polymerization initiator has been mainly used.
The conventional thermosetting resin composition containing the curable resin has not excellent mechanical properties; in particular, yellowing occurs at high temperature due to the inferior heat resistance.
Accordingly, to prevent such problems, an excessive amount of multifunctional monomer having an ethylenically unsaturated group is used, which accelerates yellowing at high temperature.
In addition, to improve mechanical properties and heat resistance, there are methods for introducing an ethylenically unsaturated group into the curable resin for the purpose of crosslinking with a crosslinking agent upon thermal curing.
In this connection, to introduce the ethylenically unsaturated group into the curable resin, methods for introducing an acrylic group in a molecule are used, such as 1) addition of glycidyl(meth)acrylate to a linear polymer copolymerized with (meth)acrylic acid, 2) addition of (meth)acrylic acid to a linear polymer copolymerized with glycidyl(meth)acrylate, 3) addition of isocyanate alkyl(meth)acrylate to a linear polymer copolymerized with hydroxyalkyl(meth)acrylate, and 4) condensation of a diol compound containing an ethylenically unsaturated group and a dianhydride compound.
However, among those methods, since the methods of 1) to 3) comprises two steps of preparing a linear polymer and introducing an acrylic group, they requires longer reaction time, and the introduced (meth)acrylic equivalent is generally within 10 wt % (wherein (meth)acrylic equivalent is defined as a percentage, calculated by dividing the amount of the introduced acrylic (CH2═CH—CO2—) or methacrylic (CH2═CCH3—CO2—) by the total weight of resin).
The method 4) also requires longer reaction time to obtain a satisfactory molecular weight, and a step of synthesizing a diol compound containing an ethylenically unsaturated group by the addition reaction of a diglycidyl compound and (meth)acrylic acid, if necessary. Further, the introduced (meth)acrylic equivalent is not above 10 wt %.
Further, in the case of using acrylic based copolymers generally used in the art as a curable resin, its molecular weight or the type of monomer is changed to control the flow properties of acrylic based copolymers. However, there is a limitation in that the structure of the acrylic based copolymer is not changed. Thus, there is a need for a resin having a completely different structure.
In the case of using a thermosetting resin such as a melamine resin, since an organic or inorganic acid or an amine or ammonium salt thereof is used in combination with a curing accelerator in the ink, these compounds remain as impurities in the film, thereby contaminating the sputtering apparatus upon formation of ITO transparent electrodes or such an ink has a short pot life when it is made as an ink in one package type.
In addition, in the case of using a solvent having a low boiling point of 200° C. or less upon manufacturing a color filter, it easily evaporates due to the characteristics of ink-jet printing method to increase the particle size of dispersed pigment, leading to a clogged printer head nozzle.
However, in the case of using a solvent having a boiling point of 200° C. or higher in order to prevent the problem, the viscosity of the solvent is increased, which is not suitable for ink. Since the problem reduces production efficiency of the color filter, it becomes a factor to impede the production efficiency, which is a major advantage of the printing method. Further, since the solvent having a relatively high boiling point has a strong intermolecular attractive force, it has a high viscosity and is not compatible with general polymer resins. Thus, there is an increasing need for the development of a solvent having a high boiling point and low viscosity as an ink solvent, and a resin which is compatible with the solvent and does not increase the viscosity of the ink composition.
During production process, the color filter is treated under severe conditions including the treatment with an organic solvent, acid, an alkali solution or the like, and heat treatment of the local surface by sputtering at high temperature. Accordingly, the curable resin used in the color filter is required to have excellent chemical resistance such as solvent resistance, acid resistance, and alkali resistance, excellent heat resistance and light resistance.